Condition controlling apparatus



July 8, 1947. J. M. WILSON ETAL 2,423,541

CONDITION CONTROLLING APPARATUS Filed Aug. 25, 1944 Patented July 8, 1947 UNITED STATES PATENT OFFICE CONDITION CONTROLLING APPARATUS John M. Wilson and Francis A. Busse, Minneapolis, Minn., assignors to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application August 25, 1944, Serial No. 551,228

16 Claims. 1

, cordance with a limitin condition, a limiting action only over a predetermined range of variation of the value of the limiting condition.

More specifically, an object of our invention is to provide an improved temperature controlling apparatus in which one resistance bulb is responsive to the value of a temperature controlling condition and another resistance bulb is responsive to the value of a limiting temperature condition, both resistance bulbs being capable of controlling the operation of the temperature changing apparatusin which means is provided when the limiting condition is on the desired side of a critical value to prevent the last-named resistance element from affecting the operation of the temperature changing apparatus.

A still further object of the present invention is to provide an arrangement of the type set forth in the previous objects in which an electronic discharge device is associated with the first impedance element and is controlled by the second impedance element, this discharge device being effective to prevent the second resistance element from affecting the operation of the condition controlling apparatus when the value of the limiting condition is on the desired side of a predetermined value.

A still more specific object of our invention is to provide an arrangement in which the first impedance element forms an element of an impedance bridge and in which the electronic discharge device is connected in parallel with the output terminals of the bridge, this discharge device being non-conductive when the value of the limiting condition is within the desired range.

Other objects of the present invention will be apparent from a consideration of the accompanying specification, claims and drawing of which sired to control.

40 ture of the furnace.

2 the single figure is a schematic showing of a temperature control system employing our improved temperature controlling apparatus.

Referring to the drawing for a more detailed 5 understanding of our invention, the condition controlling apparatus of our invention is shown as located within a housing I0 and willbe referred to hereinafter in its entirety by that re erence numeral. Associated with the apparatus In within the housing I0 and forming a part of the condition controlling apparatus is a temperature responsive resistance element I I. This element is normally located so as to be subjected to the medium whose temperature it is deof the invention illustrated in the drawing, the resistance element II is located within a space to be heated.

A condition controlling apparatus is employed to control the operation of a gas valve I2 sup- 2 which is designed to supply heat through any suitable heat transfer arrangement to the space in which the bulb I I is located. It is to be understood that there is normally associated with the pilot burner I5 some means for detecting the presence of a pilot flame and which will prevent opening of the main valve I2 until it has been assured that there is a pilot burner flame. In order to simplify the present disclosure, this means has been omitted herein.

Located within the furnace I8 is a further temperature responsive resistance element I8. This element, like element II, has a relatively high temperature coeflicient of resistance. Element I8 is located so as to respond to the tempera.- If the furnace I6, for example, is a hot water furnace the element I8 will be located so as to respond to the temperature of the boiler water. If, on the other hand, the furnace is a warm air furnace, element I8 will be located in the bonnet of the furnace or at some other suitable point where it will respond to the temperature ,of the heated air.

Referring now to the apparatus located within the housing III, the elements I I and I8 form temperature responsive elements of resistance bridges generally indicated by the reference numerals 20 and 2|, respectively. The bridge 20 comprises in addition to the temperature responsive resistance II fixed resistors 23 and 24 and an adjustable resistor 25. The resistors 23, 24 and In the particular embodiment v stitutes the other input terminal.

25 are preferably formed of a material which is relatively unaffected by temperature changes. The element II is connected by conductor 26, terminal 21 and conductor 28 to' one terminal of resistor 23. The other terminal of resistor II is connected by conductor 29 and terminal 38 to one-terminal of adjustable resistor 25. The junction of conductor 28 and resistor 23 and the junction of variable resistor 25 and resistor 24 constitute the input terminals of the bridge 28 and are designated by the reference numerals 3| and 32, respectively. The junction of resistors 23 and 24, designated by the reference numeral 33, and the terminal 38 constitute the output terminals of the bridge. The variable resistor 25 functions to adjust the output voltage for any given temperature of resistance bulb II and hence is employed to vary the temperature maintained by the apparatus.

Referring to bridge 2|, this bridge in addition to the temperature responsive resistance I8 previously mentioned includes constant resistors 35 and 36 and a manually adjustable resistor 31. None of the resistors 35, 36 and 31 is in any way affected by temperature. One terminal of resistor I8 is connected by a conductor 38, terminal 39 and conductors 48 and 4| to one terminal of resistor 38. The other terminal of resistor I8 is connected by conductor 42, terminal 43, and conductor 44 to variable resistor 31. The junction of resistors 35 and 36, designated by the reference numeral 46, constitutes one of the input terminals of the bridge, and the junction of resistor 31 and conductor 44, designated by the numeral 41, con- The junction of resistors 35 and 31 is indicated by the reference numeral 48 and the junction of conductors 48 and 4| by the reference numeral 49. Junctions 48 and 49 constitute the output terminals of the bridge.

The output of bridge 28 is employed to control the operation of a thermionic discharge device indicated by reference numeral 58. This device comprises an anode a grid 52, and a cathode 53. A cathode heater 54 is associated with the cathode 53.. The grid 52 is connected to output terminal 38 of bridge 28 by conductors 55 and 56, condenser 51 and conductor 58. The cathode 53 is connected to the output terminal 33 of bridge 28 by conductors 68. 6|, 62, 63 and 64. A resistor 66 is connected between the grid and cathode 53 and serves as a coupling resistor between the bridge 28 and the discharge device 58.

Also connected between the grid 52 and the cathode 53 is a further discharge device 68. comprises an anode 69, a grid 18, a cathode 1| and a cathode heater 12. The input circuit of this discharge ,device or tube is controlled by the output of bridge 2 I. The grid 18 is connected by conductor 14 to the output terminal 48 of the bridge, while the cathode 1| is connected by conductors 16, 62, 63 and 18, condenser 19, and conductor 88 to the output terminal 49. A resistor 82 is connected between the grid 18 and the oathode 1| by conductors 62 and 16.

The discharge device 58 controls the operation of a further discharge device 83, which comprises an anode 84, a grid 85, a cathode 86, and a cathode'gheater 81. Connected between the grid 85 and the cathode 86 of tube 83 area resistor 89, condenser 98, resistor 92, and a conductor 93. Connected in parallel with the anode and cathode of tube 58 are a resistor 95 and a condenser 96.

The discharge device 83 controls the operation of a relay 98. This relay comprises a relay i d- This ing 99 and a switch blade I88 which cooperates with a switch contact I8I. The switch blade I88 is normally biased out of engagement with contact I8I and is moved into engagement therewith upon energization of relay winding 99. A by-pass condenser I82 is connected in parallel with relay winding 99 by conductors I83 and I84. The bypass condenser I82 serves to by-Pass the A. 0. component of the current supplied by tube 83.

Power is supplied to the entire temperature control system from line wires I86 and I8! leading to any suitable source of power (not shown). Line wires I86 and I81, are connected to line voltage terminals I88 and I89 by conductors I I8 and III, respectively.

Power is supplied to the individual units of the temperature responsive apparatus by means of a transformer II2. This transformer comprises a line voltage primary 3 and a plurality of secondary windings H4, H5, H6 and H1. Secondary winding H5 is employed to energize the various cathode heaters 54, 12 and 81. The secondary H5 is connected by conductor II9, resistor I28, conductors I2I and I22, and conductors I23 and I24 to the filament heater 81. The same secondary is connected to filament heater 54 by conductor II9, resistor I 28, conductors I2I, I25 and I26, and conductors I28, I29 and I24. Filament heater 12 is connected in parallel with filament heater 54 by conductors I38 and I3I.

Secondary II4 is employed to supply power to the bridge 28, and the secondary I I1 is employed to supply power to bridge 2|. Secondary H4 is connected by conductors I 32 and I33 to the input terminals 3| and 32 of bridge 28, while secondary I I1 is connected by conductors I34 and I35 to the input terminals 46 and 41 of bridge 2|.

The primary I I3 of transformer I I2 is connected by conductors I36, I 31 and I 38 to the power supply terminals I88 and I89.

In the description to date, no mention has been made of any of the various values of the elements of the apparatus. It is to be understood that these elements may assume various values consistent with the proper operation of the apparatus. In one particular embodiment of our invention, we found it desirable to employ a transformer in which the secondaries corresponding to secondaries H4 and H1 each had a 30 voltage output and the secondary corresponding to secondary II5 had a 7 voltage output, In the same embodiment the secondary M6 was a 600 voltage secondary. In this embodiment, the two tubes 58 and 68 were combined within a single envelope and a 7F7 tube wasemployed. In the same embodiment, the tube 83 was a 7N7 tube. The resistors 82 and 66 were both 5 megohm resistors, resistor a one megohm resistor, and resistor 92 a 8.1 megohm resistor. The condensers 51 and 19 were each .01 microfarad condensers, and condensers 96 and 98 were one-half to one-fourth microfarad, respectively. The by-pass condenser I82 had a capacity of four microfarads. Resistor I42 had a resistance value of 3,008 ohms. The above values are given solely as values used in one typical embodiment of the invention and are not in any way to be construed as limiting the invention to apparatus employing elements having such values. I

Operation Under these is at or above the desired value, the bridge 28 is i very substantially unbalanced in such a direction that the potential at output terminal 30 tends to approach the potential at input terminal 32.

The input potential of terminal 32 is such that this terminal is negative with respect to terminal II'when the anode of tube 58 is positive with respect to the cathode. This will become more apparent with the subsequent description of the operations- The output terminal 38 is connected by conductor 58, condenser 51 and conductors 58 and 55, as previously explained, to the grid 52. an unbalance voltage of the bridge 28 is applied to grid 52, which unbalance voltage is out of phase with the voltage applied to the anode. The effect of this voltage is accordingly to bias the grid negative with respect to the cathode tential of input terminal 46. The input terminal 48 is connected to the upper end of secondary Ill and tends to be positive during the half cycle in which anode 5| is positive. As previously explained. during the same half cycle the grid 52 of tube 58 is negative. Since anode 69 of tube 58 is connected by conductors I5, 58 and 55 directly to grid 52 of tube 58 and since the cathodes II and'59 of tubes 58 and 88 are connected by conductors 88, 8| and I6, it will be obvious that during the half cycle being considered, anode 89 of tube 88 is negative with respect to the cathode II. It will be recalled that under the condition of unbalance existing in bridge 2|, the output terminal 48 of bridge 2| tends to be positive with respect to the output terminal 49. Hence, since the output terminal 49 is connected to cathode 'II and the output terminal 48 to grid I8, the effect of this unbalance voltage will be to apply a voltage to the grid 18 tending to make it positive during the same half cycle in which the anode 89 is negative with respect to the cathode. Thus, the alternating output voltage applied by bridge 2| to the grid I8 is out of phase with the alternating voltage across the anode and cathode of tube 88 so as to exert'a biasing effect upon the grid, Hence, the tube 58 is likewise non-conductive.

Under the conditions outlined above in which tubes- 58 and 88 are both substantially non-conductive, the grid 85 is subjected to a biasing voltage sufflcient to prevent enough current flowing through relay winding 99 to operatively energize relay 98. This is due to two effects. In the first place, a negative charge tends to be built up on that terminal of condenser 95' to which grid 85 is connected byreason of the current flowing through the following circuit: from the upper terminal of secondary I I6 through conductor I44, resistor 95, conductor I45, condenser 96, conductors I48 and I41, resistor 89, grid 85, cathode 88, conductor I48, resistor I42 and conductor I49. Due to the rectifying action of the grid and cathode, current flows through this circuit only in the direction just traced. It is to be noted that 6 this current flow is in such a direction that the upper terminal of condenser 98 tends to become positive and the lower terminal to become negative. Hence, since the grid is connected to the lower terminal of condenser 98, this tends to maintain grid 85 at a negative potential with respect to cathode 86. Furthermore, current also tends to flow through the following path: from the upper terminal of secondary I I8 through conductor I44, resistor 95, conductor I45, condenser 98, conductors I46 and I41, condenser 98, resistor 92, conductors 93 and I48, resistor I42, and conductor 449 to the center tap of secondary H8. The current flow through this path tends to impress across condenser 98 an alternatingpoten- I5| to the lower terminal of secondary I I8. Thus,

the upper terminal of secondary I I6 to which grid 85 tends to be connected by reason of the circuit just traced is at a point whose phase relationship with respect to the cathode is opposite to that of the anode. Hence, the eiTect of this alternating potential across condenser 98 is to bias the grid 85 negatively during the half cycles in which tube 83 would otherwise be conductive. As a result of these two biasing effects, the tube 83 is under these conditions maintained non-conductive with the result that relay 98 is effectively deenergized as previously mentioned.

Now let it be assumed that the temperature of the space in which resistor II is located dr p below the value it is desired to maintain. The effect of this drop is to cause the potential of terminal 38 to move toward the potential of the input terminal 3|. In other words, the amount of the unbalance of the bridge is decreased. This in turn tends to decrease the bias voltage that is applied to grid 52 so as to render tube 58 conductive. 'When this happens, a new current path is established through condenser 98 as follows: from the upper terminal of secondary II8 .through conductors I44 and I53, anode 5|, cathode 53, conductors 88, I54 and I41, condenser 98, resistor 92, conductors 93 and I48, resistor I42 and conductor I49 to the center tap of secondary IIB. Due to the fact that the tube 58 is conductive during only that half cycle in which the anode 5| is positive with respect to the cathode 53, current flow takes place through this last traced path only in the direction just traced. The direction of this current flow through condenser 98 is such as to tend to cause the lower terminal of condenser 98 to become positive with respect to the upper terminal. Due to the resistance in series with condenser 98, this charge is carried over alternate half ,cycles with no appreciable diminution. Since the grid 85 is connected to the lower terminal of condenser 98, this will raise the potential of grid 85 with respect to cathode 88 so as to cause tube 83 to become conductive during the alternate half cycles in which the anode 84 is positive. When this happens, relay winding 99 will be energized through the following circuit: from the lower terminal of secondary II6 through conductor I5I, relay winding 99, conductors I84 and I58, anode 84, cathode 88, conductor I48, resistor I42 and conductor I49 to the center tap of secondary II 6. This in turn will result in movement of the switch blade I88 into engagement with contact |8I to cause the following energizing circuit to be established to the gas valve: from 7 power supply terminal I 09 through conductors I 31- and I55, switch blade I00, contact I III, conductor I51, gas valve terminal I58, conductor I59,

gas valve I2, conductor I60, valve terminal IGI.

and conductor I62 to the other power supply terminal I08. The energization of the gas valve l2 as a result of the establishment of the circuit just traced will cause valve I2 to move to open position causing a flow of gas to the main burner I4. This gas will in turn be ignited by the pilot burner I 5. As previously noted, in an ordinary commercial embodiment of the system, some means would be provided for checking the presence of the flame at the pilot burner I before the main gas valve I2 is allowed to open. Since such a pilot checking means forms no part of the present invention, it has been omitted in order to simplify the disclosure.

The heating effect of the furnace resulting from the operation of the main burner It will eventually cause a rise in the temperature of the space in which resistor I I is located so as to again increase the imbalance of bridge 2|]. This in turn will increase the voltage between grid 52 and cathode 53 which is opposite in phase to that applied between anode 5l and cathode 53. The "grid 52 will accordingly be biased negatively so that the tube 50 will become effectively non-conductive. This in turn will interrupt the circuit previously traced through this tube and condenser 90. As a result, tube 83 will again become sufficiently non-conductive to cause the efiective deenergization of relay 98. This in turn will cause separation of switch blade I00 from contact I ill to interrupt the energizing circuit to gas valve I2 will cause the potential of terminal 48 to more nearly approach that of input terminal 41. other words, the unbalance of bridge 2| will be decreased. As previously explained, the unbal-. ance voltage of bridge 2| is normally eifective to bias the grid 10 of tube 68 sufficiently to cause tube 68 to be efifectively non-conductive. This condition will exist so long as the temperature of the furnace is below the value at which it is desired to have limit control action begin to take place. Thus, so long as the temperature of bulb I8 is below a predetermined limiting value, the tube 68 will remain non-conductive and will have no efiect upon the operation of the system.

As soon, however, as the temperature of bulb I8 rises above the predetermined selected value so as to indicate an undue rise in furnace temperature, the unbalance voltage of bridge 2| will be reduced to such a point that tube 68 will become conductive. As a result, during half of each complete cycle, current will flow through the following circuit: from the output terminal 30 of bridge 20 through'conductor 58, condenser 51. conductor 15, anode 69, cathode I I, conductors I5, 62, 63 and 64 to the other output terminal 33 of bridge 20. Due to the rectifying characteristics of tube 68, current can fiow only in the direction traced. The efiect of this current flow is to impres across condenser 57 a voltage such as to cause the left-hand terminal of condenser 57 to become negative with respect to the righthand terminal. It is to be noted that grid 52 of tube 50 is connected by conductors 55 and 56 to this left-hand terminal of condenser 51. Thus, the effect of current flow through condenser 51 and tube 68 is to drive the grid 52 negative with respect to cathode 53. This tends to cause tube to become non-conductive despite the fact that the unbalance voltage of bridge 20 may be sufficiently small as to otherwise permit tube 50 to be conductive. Thus, the tube 50 is rendered nonconductive to in turn render non-conductive tube 83 despite the fact that the temperature of the space may be sufficiently high that tubes 50 and 83 would otherwise be conductive. If the temperature of the furnace to which resistance I8, is subjected is sufficiently high, considering the temperature of space 20, tube 83 will be rendered sufliciently non-conductive to efi'ectively deenergize relay 98 and stop operation of the burner. The value of the burner temperature at which relay 98 will be effectively energized will depend, of course, upon the temperature of the space II since if the temperature of the space I I is extremely low, the tendency of tube 50 to be conductive will be sufiiciently great that a substantial rise in the resistance of bulb I8 will be neces-- sary to overcome this tendency. Furthermore, the unbalance voltage which is available for charging condenser 51 through tube 68 will be less so that the charge across condenser 51 is not as great as would be the case if the temperature of bulb II were higher. Thus, the setting of the limiting bridge 2I at which it is effective to shutdown the burner is constantly raised as the temperature of the space is lowered. This is desirable since if the temperature of the space is extremely low, it is desirable to operate the furnace at the maximum possible temperature. If, on the other hand, the temperature of the space is extremely close to the desired value, then it is desirable to shut down the system when the furnace temperature first starts entering an undesirably high range. Thus with the arrangement of our invention, it is possible to employ a resistance bulb limit control which normally has an unlimited range of operation and at the same time cause it to be operative only when the limiting temperature is above apredetermined value.

furthermore, is accomplished without the 'useof any switching mechanism responsive to furnace temperature.

Where. We have referred to tube 83 as being non-conductive, itis to be understood that this is intended merely to mean that the tube is not sufliciently conductive to supply enough current to relay coil 99 to cause switch blade I 00 to be moved into engagement with contact IDI. Actually, when the tube 83 is in the condition referred to as non-conductive, an appreciable cur- ;gnt may flow through the tube to the relay coil While we have disclosed a specific embodiment of our invention, it is to be understood that this is only for purposes of illustration and that the invention is to be limited solely by the scope of the appended claims.

We claim as our invention:

1. In condition controlling apparatus, electrical means for controlling the operation of condition changing apparatus, a first condition responsive means controlling the energization of said electri cal means and including an element responsive to the value of a, controlling condition, a second condition responsive means controlling the enerside of a. predetermined value, said second condition responsive means being operative to supply a voltage dependent in value upon the value of said limiting condition, and means responsive to the voltage supplied by said second condition responsive means for preventing said second condition responsive means from affecting said electrical means as long as the value of said limiting condition is on the desired side of said predetermined value.

2. In condition controlling apparatus, electrical means for controlling the operation of condition changing. apparatus, a first condition responsive means controlling the energization of said elec trical means and including an element responsive to the value of a, controlling condition, said first condition responsive means being operative to supply a voltage dependent in value upon the value of said'controlling condition, a second condition responsive means controlling the energization of said electrical means and including an element responsive to a limiting condition, said limiting conditionbeing of such character that a decrease in the efiect of the condition changing apparatus is desirable when the value of the limiting condition is not on the desired side of a predetermined value, said second condition responsive means being operative to supply a voltage dependent in value upon the value of said limiting condition, and means responsive to the voltage supplied by said second condition responsive means for preventing said second condition responsive means from afiecting said electrical means as long as the value of said limiting condition is on the desired side of said predetermined value.

3. In condition controlling apparatus, electrical'means for controlling the operation of condition changing apparatus, a first resistance bridge controlling the energization of said electrical means and including a resistance element responsive to the value of a controlling condition, said first resistance bridge being operative to supply an unbalance voltage dependent in value upon the value of said controlling condition, a second resistance bridge controlling the energization of saidelectrical means and including a resistance element responsive to a limiting condition, said limiting condition being of such character that'a decrease in the efiect of the condition changing apparatus is desirable when the value of the limiting condition is not on the desired side of a predetermined value, said second resistance bridge being operative to supply an unbalance voltage dependent in value upon the value of said limiting condition, and means responsive to the unbalance voltage supplied by said second resistance bridge for preventing said second resistance bridge from affecting said electrical means as long as the value of said limiting condition is on the desired side of said predetermined value.

4. In a temperature controlling apparatus, electrical means for controlling the operation of temperature changing apparatus, a first resistance bridge controlling the energization of said electrical means and including a resistance element having an appreciable temperature coefiicient of resistance and adapted to be located so as to be subject to a controlling temperature condition, said first resistance bridge being operative to supply an unbalance voltage dependent in value upon the value of said controlling temperature condition, a second resistance bridge controlling the energization of said electrical means and including a resistance element having an applreciable temperature coefflcient of resistanceand located so as to be subject to a limiting temperature condition, said limiting temperature condition being of such character that a decrease in the effect of the temperature changing apparatus is desirable when the value of the limiting temperature condition is not on the desired side of a predetermined limiting value, said second resistance bridge being operative to supply an unbalance voltage dependent in value upon the value of said limiting temperature condition, and means responsive to the unbalance voltage supplied by said second resistance bridge for preventing said second resistance bridge from affecting said electrical means as long as the value of said limiting condition is on the desired side of said predetermined value.

5. In condition controlling apparatus; a control device for controlling condition changing apparatus; means for controlling said device including an impedance element whose impedance changes with the value of a controlling condition; further means for limiting the action of said last named means whenever the value of a limiting condition is beyond a predetermined value beyond which a decrease in the effect of the condition changing apparatus is desirable, said further means comprising a further impedance element whose impedance changes with the value of said limiting condition and an electronic discharge device having an anode, a cathode, and a control element; connections between said anode and cathode and said first named means, and further connections between said control element and said further impedance element; said discharge device and said further connections being such that when said limiting condition is on the desired side of said limiting value, said discharge device is effectively non-conductive.

6. In temperature controllng apparatus; a control device for controlling temperature changing apparatus; means for controlling said device including an impedance element having an appreciable temperature coefficient of impedance and subjected to a controlling temperature condition; further means for limiting the action of said last named means whenever the value of a-limiting temperature condition is beyond a predetermined value beyond which a decrease in the effect of the temperature changing apparatus is desirable, said further means comprising a further impedance element having an appreciable temperature coemcient of impedance and subjected to said limiting temperature condition, said further means also comprising an electronic discharge device having an anode, a cathode, and. a control element; connections between said anode and cathode and said first named means; and further connections between said control element and said further impedance element, said discharge device and said. further connections being such that when said limiting temperature condition is on the desired side of said limiting value, said discharge device is eifectively nonconductive.

7. In temperature controlling apparatus; a control device for controlling temperature changing apparatus; means for controlling said device including a resistance element having an appreciable temperature coeflicient of resistance and subjected to a controlling temperature condition; further means for limiting the action of said last named means whenever the value of a limiting temperature condition is beyond a predetermined value beyond which a decrease in the effect of said further means comprising a further resistance element having an appreciable temperature coefilcient of resistance and subjected to said limiting temperature condition, said further means also comprising an electronic discharge device having an anode, a cathode, and a control element; connections between said anode and cathode and said first named means; and further connections between said control element and said further impedance element; said discharge device and said further connections being such that when said limiting temperature condition is on the desired side of said limiting value, said discharge device is effectively nonconductive.

8. In condition controlling apparatus; a control device for controlling condition changing apparatus; means for controllin said device including an impedance element whose specific impedance changes with the value of a controlling condition; connections between said impedance element and said device; means for limiting the action of said last named means whenever the value of a limiting condition is on an undesired side of a predetermined limiting value, said limiting condition being of such character that a decrease in the effect of the temperature changing apparatus is desirable when the value of the limiting condition is not on the desired side of said predetermined value, said further means comprising a further impedance element whose specific impedance changes with the value of said limiting condition and an electronic discharge device having an anode, a cathode, and a control element, connections between said anode and cathode and said connections between said impedance element and said device, and further connections between said control element and said further impedance element, said discharge device and said further connections being such that when said limiting condition is on the desired side of said limiting value, said discharge device is effectively non-conductive so as not to affect said connections between said impedance element and said device.

9. In condition controlling apparatus; a control device for controlling condition changing apparatus; means for controlling said device including an impedance network having an impedance element whose impedance changes with the value of said controlling condition and having output terminals, the voltage thereacross being dependent upon the impedance of said impedance element; an electronic discharge device having an anode, a cathode, and a control element, said anode and cathode being connected in parallel with said output terminals so that when said discharge device is conductive, said impedance network is partially shunted; and means including a further impedance element, whose impedance changes with the value of a limiting condition for applying a voltage to said control element, said limiting condition being of such character that a decrease in the effect of the condition changing apparatus is desirable when the value of the limiting condition is not on the desired side of a predetermined limiting value, said last named means being effective whenever the limiting condition has a value on the desired side of said limiting value to apply a voltage to said control element such that said discharge device is effectively non-conductive,

10. In condition controlling apparatus; a control device for controlling condition changing apparatus; means for controlling said device including an impedance network having an impedance element whose impedance changes with the value of said controllin condition and having output terminals, the voltage thereacros being dependent upon the impedance of said impedance element; an electronic discharge device having an anode, a cathode, and a control element, said anode and cathode being connected in parallel with said output terminal so that when said discharge device is conductive, said impedance network is partially shunted; and means including a second impedance network including a further impedance element whose impedance changes with the value of a limiting condition for applying a voltage to said control element, said limiting condition being of such character that a decrease in the eilfect of the condition changing apparatus is desirable when the value of the limiting condition is not on the desired side of a predetermined limiting value, said last named means being effective whenever the limiting condition has a value on the desired side of said limiting value to apply a voltage to saidcontrol element such that said discharge device is effectively non-conductive.

11. In condition controlling apparatus; a control device for controlling condition changing apparatus; means for controlling said device including a resistance bridge having a temperature responsive resistance element adapted to be located so as to respond to a controlling temperature condition and having output terminals, the unbalance voltage thereacross being dependent upon the resistance of said resistance element; an electronic amplifier connected between said output terminals and said control device; an electronic discharge device having an anode, a cathode, and a control element, said anode and cathode being connected in parallel with said output terminals between said bridge and said control device so that when said discharge device is con ductive, said bridge is partially shunted; and means including a further temperature responsive resistance element adapted to be located so as to respond to a limiting temperature condition for applying a voltage to said control element, said limiting condition being of such character that a decrease in the effect of the condition changing apparatus is desirable when the value of the limiting condition is not on the desired side of a predetermined limiting value, said last named means being effective whenever the limiting condition has a value on the desired side of said limiting value to apply a voltage to said control element such that said discharge device is effectively non-conductive.

12. In condition controllin apparatus; a control device for controlling condition changing apparatus; means for controlling said device including an impedance network having an impedance element whose impedance changes with the value of said controlling condition and having output terminals, the voltage thereacross being dependent upon the impedance of said impedance element; an electronic discharge device having an anode, a cathode, and a control element, said anode and cathode being connected in parallel with said output terminals so that when said discharge device is conductive, said impedance network is partially shunted; and means including a further impedance element whose impedance changes with the value of a limiting condition for applyin a voltage to said control element, said limiting condition being of such character that a decrease in the effect of the condition changing apparatus is desirable when the value of the limiting condition is not on the desired side of a predetermined limiting value, said last named means being elfective whenever the limiting condition has a value on the desired side of said limiting value to apply a voltage to said control element such that said discharge device is effectively non-conductive.

13. In apparatus for controlling the value of a condition, a control device for condition changing apparatus, a first resistance element whose specific resistance varies appreciably with a change in the value of a controlling condition and which is adapted to be exposed to said controlling conbeing affected by variations in the resistance value of said second resistance element without affecting the effect of said first resistance element on said control device.

15. In apparatus for controlling the temperature of a space, a control device for a furnace for dition, a second resistance element whose specific resistance varies appreciably with a change in the value of a limiting condition and which is adapted to be exposed to said limiting condition, said limiting condition being of such character that a decrease in the effect of the condition changing apparatus is desirable when the value of the limiting condition is not on the desired side of a predetermined limiting value, means including both said resistance elements for controlling said control device, and an electronic discharge device associated with said first resistance element and controlled by said second resistance element, said discharge device being operative whenever said limiting condition is on the desired side of said predetermined value to prevent said control device from being affected by variations in the resistance value of said second resistance element without affecting the eifect of said first resistance element on said control device.

14. In apparatus for controlling temperature changing apparatus, a. control devic for temperature changing apparatus, a first resistance element having a relatively high temperature coefficient of resistance and exposed to a controlling temperature condition, a second resistance element also having a relatively high temperature coeilicient of resistance and exposed to a limiting temperature condition, said limiting temperature condition being of such character that a decrease in the efiect of the temperature changing apparatus is desirable when the value of the limiting temperature condition is not on the desired side of a predetermined limiting value, means including both said resistance elements for controlling said control device, an electronic discharge device associated with said first resistance element and controlled by said second resistance element, said discharge device being onerative whenever the value of the limiting temperature is on the desired side of said predetermined value to prevent said control device from ments for controlling said control device, and an electronic discharge device associated with said first resistance element and controlled by said second resistance element, said discharge device being operative whenever the temperature of said furnace is below a predetermined value to prevent said control device from being affected by variations in the resistance value of said second resistance element without affecting the efiect of said first resistance element on said control device.

16. In controlling apparatus, a source of controlling voltage, an electrical device to be controlled, an electronic amplifier connected between said source and said device and effective to energize said device in accordance with the value of said controlling voltage, an electronic discharge device having an anode, a cathode, and a control element, said anode and cathode being connected across said source of, controlling voltage between said source and said amplifier so that when said discharge device is conductive, said source is partially shunted, and means responsive to a predetermined condition for controlling the voltage to said control element and effective as long as said predetermined condition is within a desired range of values to maintain the control element at a potential such that said discharge device is eilectively non-conductive. JOHN M. WILSON. FRANCIS A. BUSSE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,144,105 Coleman Jan. 17, 1939 2,327,690 Ackerman Aug. 24, 1943 2,272,492 Weyher Feb. 10, 1942 2,282,442 Whitiock May 12, 1942 2,370,847 Dempster Mar. 6, 1945 2,144,668 Stoessel Jan. 24, 1939 

